1. Field of the Invention
This invention generally relates to a device for developing an electrostatic latent image formed on an image bearing member, and, in particular, to a dry type developing device for developing an electrostatic latent image by applying a thin film of developer thereto. More specifically, the present invention relates to a developing device suitable for use in an imaging machine, such as an electrophotographic copier or facsimile machine.
2. Description of the Prior Art
In electrophotograph and electrostatic recording technology, an electrostatic latent image is formed on an image bearing member and it is developed into a visible image using a developing device. Such development can be carried out by application of a developer to the latent image. The developing device is called wet type when coloring particles, or often called toner, are dispersed in liquid; whereas, it is called dry type when the developer is comprised only of toner. In the dry type developing device, use is typically made of a developing sleeve, on which a thin film of toner electrically charged to a predetermined polarity is formed, for applying the toner to an electrostatic latent image.
The prior art dry type developing devices may be divided into two categories: first category using a two-component developer and second category using a mono-component developer. The two-component developer used in the first category includes a mixture of toner and carrier particles, wherein the toner particles are electrically attracted to the carrier particles, which are comprised of a magnetic material, such as iron, and thus are magnetically attracted to the peripheral surface of a developing sleeve due to a magnetic field created by magnets disposed inside of the developing sleeve. The developing devices of the second category using a mono-component developer were developed as improvements over the developing devices of the first category, and the mono-component developer includes only toner particles which are electrically and magnetically attractable. That is, the toner particles of the mono-component developer include a magnetic material as different from the toner particles of the two-component developer.
Thus, any of the prior art dry type developing device requires a developer to be magnetically attractable because of the reliance on a magnetic force for attraction of the developer onto the developing sleeve. In the case of the two-component developer, the carrier particles are magnetically attracted to the developing sleeve and the toner particles are electrically attracted to the carrier particles, and the toner particles are electrostatically transferred to the image bearing member selectively in accordance with the charge pattern of an electrostatic latent image formed on the image bearing member. On the other hand, in the case of the mono-component developer, since the toner particles themselves include a magnetic material, the toner particles are magnetically attracted to the peripheral surface of the developing sleeve and then they are selectively transferred to the image bearing member according to an electrostatic force acting between the toner particles and an electrostatic latent image formed on the image bearing member. Because of the necessity to create a magnetic field, one or more magnets must be disposed inside of the developing sleeve, which tends to make the developing sleeves large in size. Besides, the provision of magnets inside of the developing sleeve presents some difficulty in manufacture, in particular in an assembling process. Therefore, there has still been a need to develope an improved device for developing an electrostatic latent image.